In wireless telecommunication systems multiple carriers are coupled to an amplifier whose output is connected to an antenna for transmission. A conventional system is shown in FIG. 1 which shows three of N (S1, S2, SN) signals coupled to respective digital-to-analog converters (DACs) identified as DA1, DA2 and DAN. The output (SBi) of each DAC is coupled to an input of a respective modulator (MDi). Each modulator (MDi) has another input coupled to a corresponding carrier wave generator (CGi). The outputs (SC1, SC2, SCN) of the modulators are then supplied to summing network 20 for combining the SCi signals. The output of summer 20 is applied to the input of power amplifier 30 whose output is supplied to antenna 40 for transmission. Amplifier 30 amplifies and transmits these multiple modulated carriers (e.g., SC1, SC2, SCN). Amplifier 30 will generate unacceptably high intermodulation products (because of amplifier nonlinearity and peaking) unless amplifier 30 is operated with an average output power which is considerably less than the maximum output power. A problem with operating amplifier 30 significantly below its maximum power capability is that it results in an undesirable loss in efficiency.
Where multiple carriers are coupled to the input of an amplifier, intermodulation occurs at the amplifier output because of two factors: (1) amplifier nonlinearity; and (2) "peaking" of the carrier signals.
The gain of an amplifier is nonlinear because the amplifier has a maximum output power and the amplifier saturates as its output power gets closer to its maximum, as shown in transfer characteristic A of FIG. 2. Typically, the output power of the amplifier is modelled as the error function of the input power. This nonlinear characteristic of the amplifier results primarily in third order intermodulation products which become larger as the amplifier output power approaches saturation. If the amplifier characteristic is known, then based on the input signal power, the amplifier response power of the signal into the amplifier can be adjusted to linearize the amplifier up to the peak output power, i.e., the amplifier can approximate an ideal limiting amplifier having a transfer characteristic of the type shown as B of FIG. 2.
The response of the amplifier due to its nonlinearity problem is greatly affected by "peaking". "Peaking" as used herein refers to the phase alignment of the multiple carrier signals such that there is addition of the amplitude of the input signals, as shown in FIG. 3A. With phase alignment of the multiple carrier modulated input signals to the amplifier, the "peak" input signal power can be many times greater than the "average" input signal power. Due to amplifier nonlinearity, when peaking occurs, the intermodulation of the signals produced at the output of the amplifier is exacerbated.
For example, consider N carrier input signals applied to an amplifier, each carrier having a maximum amplitude into the amplifier of one (1). The average power of each input signal is then 1/2, and the total input power into the amplifier averaged over all possible phases for these N signals is N/2. However, if all the signals are aligned in phase, then the "total" input signal amplitude of the N signals is N and the input power is N.sup.2 /2. Thus, the phase alignment of the carrier input signals, i.e., "peaking", can increase the input signal power by up to N times. Without any correction for peaking, the amplifier must operate at an average output power of 1/N of its maximum output power to avoid peaking and resultant intermodualtion; this condition is highly inefficient.
A known method to reduce intermodulation includes a feedforward, postcompensation technique outlined in FIG. 4A. As illustrated in FIG. 4A, based on the combined input signal to amplifier 30, a signal is generated via analog network 50 that is subtracted from the amplifier output in summer 60 to significantly reduce intermodulation. This technique can typically permit the average input power to be within 5 dB of the peak input power of the amplifier with acceptable intermodulation, but requires extensive analog circuitry in networks 50 and 60 and is very sensitive to amplifier gain variation.
Another method to reduce intermodulation, employs a feedback precompensation technique, illustrated in FIG. 4B. Based on the output signal, a feedback signal is generated via feedback network 70 that is added via summer 80 to the combined input signal (ST) to reduce the joint effect of peaking and amplifier nonlinearity.
The systems shown in FIGS. 4A and 4B can be used to reduce intermodulation resulting from amplifier nonlinearity and from the input signal peaking. However, the prior art systems of FIGS. 4A and 4B require analog circuitry which can be costly. They also require significant power and hardware, and are dependent on the accuracy of the analog circuitry, which is highly undesirable. Consequently, the performance of other approaches needs to be investigated to reduce the problem of intermodulation.
In an article entitled Some Properties of Multiple Carriers and Intermodulation by Michael J. Gans, pp. 883-886, IEEE Vehicular Technology Conference, '93 ("Gans"), it is shown that by providing a fixed phase adjustment to a large number of unmodulated carriers coupled to an amplifier, the amplifier can be operated at an average power which is within 3 dB of the maximum power. However, it is harder to "depeak" (i.e., reduce the "peaking" of) the output signal if there are fewer unmodulated carriers. Hence, this value (i.e., 3 dB) increases with fewer carriers. By way of example, with 7 carriers the average power is only within 4 dB of the maximum power.
More importantly, Gans teaches using fixed phase adjustments where the input signals to the system are unmodulated. Referring to FIG. 1, for purpose of illustration, this means that the values of S1 through SN would be held constant and fixed phase adjustments made for the carrier signals to reduce peaking.